SSH to the ESXi host with root credentials and run the following commands:

/etc/init.d/hostd stop
/etc/init.d/vpxa stop
/etc/init.d/hostd start

Wait a couple of minutes for the hostd to start, then ESXi should be able to be used as a destination for converstions.

On completion of the conversions, run the following command to start the vCenter agent:

/etc/init.d/vpxa start

References

vCenter Converter Standalone 5.0 errors when an ESXi 5.0 host is selected as a destination: The access to the host resource settings is restricted. Use the management server as a destination (2012310)
http://kb.vmware.com/kb/2012310